Methods for manufacturing glass substrates by employing down-draw processing have conventionally been used. In down-draw processing, molten glass is first poured into a forming member, and then, the molten glass is made to overflow from the forming member. The molten glass then flows downward along the forming member. The streams of the molten glass merge at the lower end of the forming member, and the molten glass then separates from the forming member and is made into a sheet-form glass (sheet glass). The sheet glass is cooled by the atmosphere inside a furnace while flowing downward. The sheet glass is then cut into desired sizes, and is further processed and finally made into glass substrates.